Optical modulator and method of manufacturing same

ABSTRACT

Laser light emitted from a vertically confined surface emitting laser (VCSEL) is incident on a side surface near an end region of an optical waveguide. The end region of the optical waveguide is processed by polishing to taper at an angle of 45 degrees, and an optical modulator is formed on the polished surface. The optical modulator is a Fabry-Perot modulator using a linear electro-optical effect. The modulator has a thick transparent electro-optical layer which is deposited by using an aerosol deposition method.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a divisional application of U.S. patentapplication Ser. No. 11/091,134 filed on Mar. 28, 2005.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to an optical modulator which modulateslight from a semiconductor laser or light source at high speeds fortransmission, and more particularly, to such an optical module for usein high-speed, large-capacity optical communications and opticalinterconnections.

(2) Description of the Related Art

Optical communication networks have enabled communication of largequantities of information. However, according to the estimates ofscholars, the communication infrastructure in Japan will experienceshortages of capacity due to rapid increases in the amount of datacommunications and will become incapable of responding to the demandwithin five years, at the earliest. It is said that the tendency forcapacity shortage will be accelerated by the proliferation of broadbandcommunications and mobile telephone communications including highquality moving images.

At present, Ethernet (registered trademark) is widely used as acommunication protocol which supports the Internet protocol (IP),however, current communication systems have just begun practicalcommunications at 10 Gbps. Imminent requirements exist for thedevelopment of communication technologies which provide communication at40 Gbps or higher for inter-city (metro) communications and the likewhich involve heavy communication traffic, though not over largedistances.

In addition, at present, optical modulation which directly modulates thecurrent of a semiconductor laser (e.g., laser diode (LD)) that is alight source is widely employed in optical transmitters because of itssimple configuration. For marine optical cable communications whichinvolve long distance transmissions, external optical modulators areemployed because the charping of transmitted light caused by the directmodulation of LD causes degradation in waveforms due to the relationshipto group velocity dispersion in optical fibers.

Modulation frequencies beyond 10 GHz cause many optical components tohave poor performance. LD-based direct modulation produces a delay inthe change in the density of an injected carrier over time, thusencountering difficulties in modulation at a relaxation oscillationfrequency or higher. An external optical modulator that uses aferroelectric crystal experiences difficulties in matching the outputimpedance of a driving source due to a lower impedance of the device,thereby resulting in a failure in providing sufficient modulationcharacteristics.

Likewise, in electronic circuits and LSIs for processing digitalsignals, the transmission of signals at higher speeds results indifficulties in the transmission of signals through metal wiresparticularly between circuits and between chips, so that there is animmediate need for the development of new technologies for optical fiberlines, optical interconnections, and the like. In this case, sincemultiple optics are integrated in a chip, another essential requirement,in addition to improved speed, is reduced size.

From the foregoing, it is no exaggeration to say that a key to enablingoptical communication and optical wiring technologies at communicationspeeds exceeding 40 Gbps is to develop small and low-cost opticalmodulators.

Mach-Zehnder waveguide optical modulator is one of the opticalmodulators, at present, that have a practical use. This modulator relieson an electro-optical effect to provide a phase difference betweenwaveguided lights which propagate through two arms that are branched ina Y-shape. As the waveguided lights which have propagated through botharms are combined again at a Y-shaped branch, the intensity of thecombined light is modulated in accordance with the provided phasedifference. This modulator employs a Ti thermally diffused waveguidemade of electro-optical crystals of ferroelectric lithium niobate.

An expensive optical modulator for optical communications, in which anintensive optical modulator and an edge emission distributed feedbacklaser are monolithically integrated, is provided. The intensive opticalmodulator has advantage of an electro absorption effect in which aninverse bias is applied to an optical semiconductor crystal, aFrantz-Keldish effect, or a quantum confined Shutark effect (QCSE).

On the other hand, up to the present, the optical path length n·d (wheren is the refractive index, and d is a geometric thickness) of aFabry-Perot Ethalon surface resonator was actively changed to controlthe wavelength sweeping and the transmission wavelength of a filter byrepeated multiple reflection interference. If the wavelength of incidentlight is fixed, this operation corresponds the optical modulation of anoptical modulator.

An example of such optics is a variable wavelength filter made of anoptical semiconductor material as disclosed in JP-10-136116-A (see pages3-4 and FIG. 4). This wavelength filter, which includes a multi-quantumwell (MQW) layer sandwiched by highly reflective films, varies thewavelength which is transmitted by the filter by applying an electricfield between the reflective films to change the refractive index of theMQW layer.

JP-10-136116-A also describes the utilization of a change in therefractive index through a carrier plasma effect and a band fillingeffect which can be produced through current injection.

In addition, JP-61-088229-A (see pages 2-3 and FIGS. 1-3) disclosesanother example of a variable wavelength filter made of a transparentdielectric material. This variable wavelength filter is composed of aquartz glass substrate; dielectric multi-layer light reflection filmseach made of TiO₂ and SiO₂; a polycrystalline film made of PLZT (Ladoped oxide zirconium, oxide titanium and oxide lead compound) in athickness of approximately 200 nm which has an electro-optical effect;and a dielectric multi-layer film identical to the foregoing. Thesecomponents are sequentially deposited on the quartz glass substrate by asputtering method to form an Ethalon resonator. The wavelengthtransmitted by the filter is controlled by applying a voltage betweenthe reflection films, making use of a change in the refractive indexwhich is assumed to be attributable to a secondary electro-opticaleffect (Kerr effect).

This device encounters difficulties in forming a thick ferroelectricfilm to increase voltage sensitivity. On the other hand, theferroelectric film can be deposited by a CVD (chemical vapor deposition)method, vapor deposition, a sol/gel method, and a method ofmanufacturing a composite structure as disclosed by JP-2002-235181-A(see page 16 and FIGS. 11, 13-14), other than the sputtering method.

However, any of the prior art optical modulators described above cannotsuccessfully meet all requirements for application in opticalcommunication and optical wiring technologies at 40 Gbps or higher suchas faster operation, reduction in driving voltage, reduction in size, anarrayed arrangement, lower cost, and the like.

A Mach-Zehnder optical modulator which employs a Ti thermally diffusedwaveguide made of electro-optical crystals of ferroelectric lithiumniobate has dimensions on the order of several centimeters, and a longdevice length because of a small specific refractive index differenceand a small waveguide turnout angle. Also, the Mach-Zehnder opticalmodulator requires a high driving voltage because of a smallelectro-optical constant presented by the material. Further, theMach-Zehnder optical modulator has limitations when operating at 20 GHzbecause of difficulties in matching a characteristic impedance of atravelling wave electrode, which is applied with a modulation signal,with a driving source.

The lumped circuit type optical modulator that takes advantage of anelectro absorption effect with an inverse bias applied to an opticalsemiconductor crystal, or a quantum confined Shutark effect, entails ahigh manufacturing cost because of the requirement for advanced andcomplicated compound semiconductor crystallization technologies andlithography. This makes the lumped circuit type optical modulatorunsuitable for applications in optical wiring which often involves aplurality of elements arranged in array.

The optical modulator device (JP-01-136116-A) which includes awavelength filter made up of a multi-quantum well (MQW) layer of anoptical semiconductor sandwiched by highly reflective films and whichvaries a wavelength transmitted by the filter, entails a high costbecause the substrate is limited to crystals which match the conditionof crystal growth layer in lattice and there is a need for a crystalgrowth technology based on advanced apparatuses. Also, difficulties informing a thick MWQ layer inevitably contribute to a lower sensitivity.Further, since the optical modulator device utilizes the red shift of afundamental absorption end near the transmission wavelength caused by anapplied voltage, a change in the refractive index is accompanied by acoincident change in light absorption, causing the device to invariablysuffer from a high light insertion loss. If the blue shift by carrierinjection is utilized, noise light, such as naturally emitted light, isgenerated in addition to a coincidental change in light absorption.Consequently, the technologies described in JP-01-136116-A are noteffective for application in light control devices such as an opticalmodulator, a variable wavelength filter and the like.

In the optical modulator device (JP-61-088229-A) which includes anEthalon resonator made of PLZT having an electro-optical effect on aquartz glass substrate, it is difficult to form a thick ferroelectricfilm to increase the sensitivity to the voltage. Supposing that theoptical modulator device includes a thick ferroelectric film, thequality of the film would be degraded, resulting in increased lightscattering, worsened spatial coherence of light, and increased lightloss. Also, since the optical modulator device is estimated to utilizethe Kerr effect, the device has a problem of poor controllability due tohysteresis introduced in the voltage versus light transmissioncharacteristic.

The sputtering method used for depositing the ferroelectric material,described in JP-61-088229-A, can produce an optically high quality filmin a thickness of 2 μm or less at most. Also, the manufacturing methoddisclosed in JP-2002-235181 for depositing a ferroelectric film hasnever successfully produced an optically high quality film. In order toincrease the impedance and reduce the voltage of the Ethalon resonatortype optical modulator, it is necessary to increase the film thicknessfive times larger than can be done by conventional methods.

SUMMARY OF THE INVENTION

The present invention has been made in view of the problems experiencedby the conventional optical modulators, and it is an object of theinvention to provide a highly sensitive and compact optical modulatorwhich facilitates impedance matching with a driving source, has lightmodulation characteristics including ultra-high speeds exceeding 40 Gbpsand a low driving voltage, and can be utilized in optical wiring and thelike, and to provide a method for manufacturing such an opticalmodulator.

It is another object of the present invention to provide a opticalmodulation system which employs the optical modulator described above,as well as an optical interconnect device and an optical communicationdevice which use the optical modulation system.

The present invention has been made based on the knowledge thatrealizing the operation of an optical modulator at an ultra-highfrequency and with a low driving voltage, can be done by increasing theimpedance of the device at high frequencies beyond the impedance of adriving source, as described above. In addition, the present inventionaims at increasing the thickness of an electro-optical layer inaccordance with the basic idea for accomplishing the operation with alow driving voltage.

An optical module according to one aspect of the present invention isdriven by an external driving source, and is characterized by includinga Fabry-Perot resonator type optical modulation unit formed on asubstrate, wherein the optical modulation unit includes a laminatestructure comprised of an electro-optical layer, an upper electrodelayer overlying the electro-optical layer and a lower electrode layerunderlying the electro-optical layer to sandwich the electro-opticallayer therebetween, and an upper reflective layer overlying the upperelectrode layer and a lower reflective layer underlying the lowerelectrode layer to sandwich the upper electrode, electro-optical layer,and lower electrode layer therebetween, and the optical modulation unithas a reactance, the absolute value of which is higher than theimpedance of the external driving source in the frequency range equal toor lower than 200 GHz.

According to the foregoing configuration, since the impedance of theoptical modulator, which is a capacitive load, is high in a highfrequency range as well, the output voltage of the driver can respond tothe modulation at high frequencies. Consequently, the refractive indexof the electro-optical layer can also be changed at high speeds, therebyenabling high-speed modulation.

In the optical modulator described above, the electro-optical layer mayhave a linear electro-optical effect.

An optical modulator according to another aspect of the presentinvention is driven by an external driving source, and is characterizedby including a Fabry-Perot resonator type optical modulation unit formedon a substrate, wherein the optical modulation unit includes a laminatestructure comprised of an electro-optical layer, an upper electrodelayer overlying the electro-optical layer and a lower electrode layerunderlying the electro-optical layer to sandwich the electro-opticallayer therebetween, and an upper reflective layer overlying the upperelectrode layer and a lower reflective layer underlying the lowerelectrode layer to sandwich the upper electrode, electro-optical layer,and lower electrode layer therebetween, and the electro-optical layerhas a linear electro-optical effect.

In each optical modulator, the electro-optical layer may be made of asingle crystal or a sintered compact. Also, the electro-optical layermay be formed by a deposition method. Further, the electro-optical layermay be deposited by an aerosol deposition method. Since the aerosoldeposition can deposit a film in a thickness of several μm or more, theelectro-optical layer can be deposited in a large thickness. As aresult, a lower driving voltage can be applied to the optical modulatorto reduce the capacitance and increase the impedance. The composition ofthe electro-optical layer may include either lead zirconate titanate, orlead zirconate titanate added with lanthanum, or KTN (kalium tantalumniobate). The optical modulator may be formed on a curved surfacearranged on the substrate. The curved surface may be a concave.

An optical modulation system according to one aspect of the presentinvention includes a light source, an optical modulator driven by anexternal driving source for modulating light emitted from the lightsource, and an optical waveguide for guiding modulated light modulatedby the optical modulator, and is characterized in that the opticalmodulator is a Fabry-Perot resonator type optical modulator formed on asubstrate, which includes a laminate structure comprised of anelectro-optical layer, an upper electrode layer overlying theelectro-optical layer and a lower electrode layer underlying theelectro-optical layer to sandwich the electro-optical layertherebetween, and an upper reflective layer overlying the upperelectrode layer and a lower reflective layer underlying the lowerelectrode layer to sandwich the upper electrode, electro-optical layer,and lower electrode layer therebetween, and the optical modulator has areactance, the absolute value of which is higher than the impedance ofthe external driving source in the frequency range equal to or lowerthan 200 GHz.

A optical modulation system according to another aspect of the presentinvention includes a light source, an optical modulator for modulatinglight emitted from the light source, and an optical waveguide forguiding modulated light modulated by the optical modulator, and ischaracterized in that the optical modulator is a Fabry-Perot resonatortype optical modulator formed on a substrate, which includes a laminatestructure comprised of an electro-optical layer, an upper electrodelayer overlying the electro-optical layer and a lower electrode layerunderlying the electro-optical layer to sandwich the electro-opticallayer therebetween, and an upper reflective layer overlying the upperelectrode layer and a lower reflective layer underlying the lowerelectrode layer to sandwich the upper electrode, electro-optical layer,and lower electrode layer therebetween, and the electro-optical layerhas a linear electro-optical effect.

In each optical modulation system, the optical waveguide may have an endsurface inclined with respect to the optical axis of the opticalwaveguide, the optical modulator may be formed on the inclined surface,and the light source may be disposed such that the light emittedtherefrom is transmitted through a side surface of the optical waveguideand is incident on the optical modulator. Alternatively, the end surfaceof the optical waveguide may be vertical to the optical axis of theoptical waveguide, the optical modulator may be formed on the verticalsurface, and the light source may be disposed such that the lightemitted therefrom is incident on the optical modulator at right angles.The optical modulation system may further include a lens for couplingthe light emitted from the light source to waveguided light of theoptical waveguide. The optical modulation system may further include anoptical modulator support having a reflective surface for converting theemission optical axis of the light source to the optical axis of theoptical waveguide, wherein the optical modulator may be formed on thereflective surface of the optical modulator support. The reflectivesurface may be a concave. The concave may have a curvature for focusinglight emitted from the light source on the optical waveguide.

An optical interconnect device according to one aspect of the presentinvention has a optical modulation system including a substrate, a lightsource having an emission optical axis vertical to the surface of thesubstrate, an optical waveguide having an optical axis parallel with thesurface of the substrate, and an optical modulator, and is characterizedin that the optical modulation system is any of the optical modulationsystems according to the aspects of the present invention describedabove, the optical waveguide has an end surface inclined with respect tothe optical axis of the optical waveguide, the optical modulator isformed on the inclined surface, and the light source is disposed suchthat the light emitted therefrom is transmitted through a side surfaceof the optical waveguide and is incident on the optical modulator.

An optical interconnect device according to another aspect of thepresent invention has a optical modulation system including a substrate,a light source having an emission optical axis vertical to the surfaceof the substrate, an optical waveguide having an optical axis parallelwith the surface of the substrate, and an optical modulator, and ischaracterized in that the optical modulation system is any of theoptical modulation systems according to the present invention describedabove, and includes an optical modulator support having a reflectivesurface for converting the emission optical axis of the light source tothe optical axis of the optical waveguide, and the optical modulator isformed on the reflective surface of the optical modulator support.

An optical communication device according to one aspect of the presentinvention has a optical modulation system including a substrate, a lightsource having an emission optical axis vertical to the surface of thesubstrate, an optical waveguide having an optical axis parallel with thesurface of the substrate, and an optical modulator, and is characterizedin that the optical modulation system is any of the optical modulationsystems according to the present invention described above, the opticalwaveguide has an end surface inclined with respect to the optical axisof the optical waveguide, the optical modulator is formed on theinclined surface, and the light source is disposed such that the lightemitted therefrom is transmitted through a side surface of the opticalwaveguide and is incident on the optical modulator.

An optical communication device according to another aspect of thepresent invention has a optical modulation system including a substrate,a light source having an emission optical axis vertical to the surfaceof the substrate, an optical waveguide having an optical axis parallelwith the surface of the substrate, and an optical modulator, and ischaracterized in that the optical modulation system is any of theoptical modulation systems according to the present invention describedabove, and includes an optical modulator support having a reflectivesurface for converting the emission optical axis of the light source tothe optical axis of the optical waveguide, and the optical modulator isformed on the reflective surface of the optical modulator support.

A method of manufacturing an optical modulator according to the presentinvention is characterized by including the steps of, forming aFabry-Perot resonator type optical modulator on a substrate, wherein theoptical modulator includes a laminate structure comprised of anelectro-optical layer, an upper electrode layer overlying theelectro-optical layer and a lower electrode layer underlying theelectro-optical layer to sandwich the electro-optical layertherebetween, and an upper reflective layer overlying the upperelectrode layer and a lower reflective layer underlying the lowerelectrode layer to sandwich the upper electrode, electro-optical layer,and lower electrode layer therebetween, and determining the thickness ofthe electro-optical layer such that an absolute value of a reactanceformed by the optical modulator is higher than the impedance of anexternal driving source for driving the optical modulator in thefrequency range equal to or lower than 200 GHz.

Another method of manufacturing an optical modulator according to thepresent invention is characterized by including the steps of, forming aFabry-Perot resonator type optical modulator on a substrate, wherein theoptical modulator includes a laminate structure comprised of anelectro-optical layer, an upper electrode layer overlying theelectro-optical layer and a lower electrode layer underlying theelectro-optical layer to sandwich the electro-optical layertherebetween, and an upper reflective layer overlying the upperelectrode layer and a lower reflective layer underlying the lowerelectrode layer to sandwich the upper electrode, electro-optical layer,and lower electrode layer therebetween, and developing a linearelectro-optical effect possessed by the electro-optical layer.

In the respective manufacturing methods described above, theelectro-optical layer may be made of a single crystal or a sinteredcompact. Also, the electro-optical layer may be formed by a depositionmethod. Further, the electro-optical may be deposited by an aerosoldeposition method.

As will be appreciated from the foregoing, the present invention canrealize a highly sensitive and compact optical modulator and opticalmodulation system which have ultra-high speed optical modulationcharacteristics, and can be utilized even in optical wiring and thelike. Moreover, the present invention can provide an opticalinterconnect device and an optical communication device which employ theoptical modulation system as described above.

The above and other objects, features, and advantages of the presentinvention will become apparent from the following description withreference to the accompanying drawings which illustrate examples of thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the configuration of a first embodimentof a optical modulation system according to the present invention;

FIG. 2 is a diagram illustrating the structure, viewed in cross-section,of a first example of an optical modulator according to the presentinvention;

FIG. 3 is a top plan view of the first example of the optical modulatoraccording to the present invention;

FIG. 4 is a graph showing the dependence of the reflectivity on theapplied voltage of the optical modulator in the first examplemanufactured according to the present invention;

FIG. 5 is a diagram illustrating the structure, viewed in cross-section,of a second example of the optical modulator according to the presentinvention;

FIG. 6 is a graph showing the dependence of the reflectivity on theapplied voltage of the optical modulator in the second examplemanufactured according to the present invention;

FIG. 7 is a diagram illustrating the structure, viewed in cross-section,of a third example of the optical modulator according to the presentinvention;

FIG. 8 is a graph showing the dependence of the reflectivity on theapplied voltage of the optical modulator in the third examplemanufactured according to the present invention;

FIG. 9 is a graph showing the transmission spectrum of a sample forclarifying the effect of a lower intermediate layer;

FIG. 10 is a graph showing the transmission spectrum of a sample forclarifying the effect of polishing an electro-optical layer manufacturedby an aerosol deposition method;

FIG. 11 is a diagram illustrating the structure, viewed incross-section, of a fourth example of the optical modulator according tothe present invention;

FIG. 12 is a diagram illustrating the configuration of a secondembodiment of the optical modulation system according to the presentinvention;

FIG. 13 is a diagram illustrating the configuration of a thirdembodiment of the optical modulation system according to the presentinvention;

FIG. 14 is a diagram illustrating the configuration of a fourthembodiment of the optical modulation system according to the presentinvention;

FIG. 15 is a diagram illustrating the configuration of a fifthembodiment of the optical modulation system according to the presentinvention;

FIG. 16 is a graph showing the dependence of the reflectivity on theapplied voltage of the optical modulator manufactured in the fifthembodiment of the optical modulation system according to the presentinvention; and

FIG. 17 is a diagram illustrating the configuration of one embodiment ofan interconnection according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

FIG. 1 illustrates the configuration of a first embodiment of a opticalmodulation system which includes an optical modulator according to thepresent invention. Laser light 11 emitted from vertically confinedsurface emitting laser (VCSEL) 10 is incident on a side surface near anend of optical waveguide 12. Optical waveguide 12 comprises core layer13, and cladding layer 14 surrounding core layer 13. An end region ofoptical waveguide 12 is processed by polishing to taper at an angle of45°, and optical modulator 15 is formed on the polished surface. Opticalmodulator 15 is placed at such a position that light reflected therebyenters core layer 14 of optical waveguide 12.

Optical modulator 15 comprises a Fabry-Perot resonator. Light incidenton optical modulator 15 repeatedly reflects within and interferes withoptical modulator 15, causing a change in the intensity ratio ofstraight ahead transmitted light to 90° reflected light in accordancewith an optical path length n·d (where n is the refractive index, and dis a geometrical thickness) within optical modulator 15. When opticalmodulator 15 is not applied with any voltage, the optical path lengthd·n within optical modulator 15 is set such that the straight aheadtransmitted light of the incident laser light has an intensity equal toone at the wavelength at which VCSEL 10 oscillates, while the 90°reflected light has the intensity equal to zero. As optical modulator 15is applied with a voltage, the refractive index n changes due to theelectro-optical effect which changes the intensity of the straight aheadtransmitted light of the incident laser light to zero, and which changesthe intensity of the 90° reflected light to one, so that the incidentlight is fully introduced into core 13 of optical waveguide 12. Withthis operation, the application of laser light to waveguide 12 can beturned on and off.

Alternatively, the optical path length of optical modulator 15 can beset such that the intensity of the 90° reflected light changes to onewhen no voltage is applied to optical modulator 15. In this way, anoptical bias can be arbitrarily set.

To increase the coupling efficiency of laser light 11 to opticalwaveguide 12, an anti-reflection film is preferably formed on the lighttransmission surface of optical waveguide 12. Also, a focusing lens canbe disposed between VCSEL 10 and optical modulator 15 as required.

Also, while optical modulator 15 is formed in an end region of opticalwaveguide 12 in the first embodiment, an end region of an optical fibermay be processed to have a tapered surface on which optical modulator 15can be formed. In addition, the light source is not limited to VCSEL,but an edge emitting laser, LED, and the like can be used for the lightsource.

[First Example of Optical Modulator]

FIG. 2 illustrates the structure, viewed in cross-section, of opticalmodulator 15 which is a first example of the optical modulator accordingto the present invention. The following description will focus on amethod of manufacturing this optical modulator.

First, dielectric multi-layer reflective layer 20 is formed by an ionplating method. Lower dielectric multi-layer reflective layer 20 iscomposed of many layers of an SiO₂ film (303 nm thick) and a Ta₂O₅ film(186 nm thick) which are alternately deposited one on the other. Here,the thickness was controlled during the deposition by opening andclosing a shutter on a deposition source while measuring the opticalspectrum on a monitor.

Next, lower electrode layer 21, i.e., an ITO thin film which is highlytransparent at the wavelength of laser light emitted by VCSEL isdeposited by a sputtering method. Specifically, the ITO thin film isdeposited using In₂O₃—SnO₂ (10 wt %) as a target and a mixture of argonand oxygen as a sputtering gas. The temperature on a substrate is 250°C., and the ITO thin film is deposited in a thickness of 235 nm. Otherthan ITO, lower electrode layer 21 can be made, for example, of a lowelectric resistance material such as ZnO which is transparent at thewavelength of laser light from VCSEL. In any case, it is important toincrease the transmittance at the wavelength of laser light from VCSELand to reduce the electric resistance by controlling the depositioncondition, composition, and carrier concentration.

Next, lower intermediate layer 22 is formed on lower electrode layer 21by an ion plating method. Lower intermediate layer 22 is formed forpurposes of reducing the influence of light scattering caused by a roughinterface which is formed when electro-optical layer 23 is deposited,and is required to have a thickness of 20 nm or more. In this example,the thickness of lower intermediate layer 22 was chosen to be 100 nm.Also, for preventing Fresnel reflection on the interface between lowerintermediate layer 22 and electro-optical layer 23, lower intermediatelayer 22 is required to have a refractive index close to or equal tothat of electro-optical layer 23. In this example, lower intermediatelayer 22 is made of Ta₂O₅ film.

An aerosol deposition method, in which an ultra-fine particle fragilematerial is crushed by applying the same with a mechanical impact force,and crushed particles are bonded to form a molded structure, is used toform electro-optical layer 23. Electro-optical layer 23 has a thicknessof 6000 nm. Pb(Zr0.6Ti0.4)O₃ is used as raw powder, and nitrogen is usedas a carrier gas. The incident angle of a nozzle to the substrate is 10degrees, the amount of gas flow is 12 liters/minute, the distancebetween the nozzle and the substrate is 5 mm, the deposition rate is 0.8μm/minute, and a vibrator vibrates at 250 rpm. Electro-optical layer 23is deposited under the foregoing conditions.

After the deposition of electro-optical layer 23, heat treatment isperformed at 600° C. for approximately 15 minutes in the atmospherewhile applying an electric field of approximately 100 kV/cm to developthe linear electro-optical effect of electro-optical layer 23. In thisevent, the primary electro-optical coefficient r33 was 200 pm/V. Afterheat treatment, the surface of electro-optical layer 23 is flatlypolished until the thickness is reduced to 5400 nm in order to removeruggedness on the surface of electro-optical layer 23.

Upper intermediate film 24 is deposited on the flat surface ofelectro-optical layer 23 by an ion plating method. Upper intermediatelayer 24 is formed for purposes of adjusting the optical thicknessbetween reflective layers required to build a Fabry-Perot resonator,i.e., the sum total of the optical thicknesses of lower intermediatelayer 22, electro-optical layer 23, and upper intermediate layer 24.Here, upper intermediate layer 24 was deposited so as to have apredetermined thickness while measuring the optical spectrum on amonitor during the deposition. Upper intermediate layer 24 is requiredto have a refractive index close to or equal to that of electro-opticallayer 23 for preventing Fresnel reflection on the interface betweenupper intermediate layer 24 and electro-optical layer 23. In thisexample, a Fabry-Perot resonator was formed by laminating upperintermediate layer 24, which was a Ta₂O₅ film (80 nm thick), onelectro-optical layer 23.

Next, an ITO thin film, which serves as upper electrode layer 25, isdeposited on upper intermediate layer 24 by a sputtering method. Upperelectrode layer 25 has a thickness of 235 nm. The deposition conditionsare the same as those for lower electrode layer 21.

Finally, upper dielectric multi-layer reflective layer 26 is formed onupper electrode layer 25 by an ion plating method. Upper dielectricmulti-layer reflective layer 26 is formed by repeatedly laminating anSiO₂ film (303 nm thick) and a Ta₂O₅ film (186 nm thick). Here, thethicknesses were controlled during the deposition by opening and closingthe shutter on the deposition source while measuring the opticalspectrum on a monitor.

While in the foregoing description, electro-optical layer 23 has thecomposition represented by Pb(Zr0.6Ti0.4)O₃, electro-optical layer 23 inthe present invention is not limited to this composition. For example,electro-optical layer 23 may be made of a single crystal or sinteredcompact. Also, electro-optical layer 23 may additionally include La inits composition. Further, other than lead zirconate titanate basedmaterials, KTN(KTaNbO₃), BaTiO₃ and the like, which exhibit largeelectro-optical effects, are effective materials for electro-opticallayer 23. However, any material having high electro-opticalcharacteristics can be used for electro-optical layer 23, and are notlimited to the foregoing described materials.

FIG. 3 is a top plan view of the optical modulator. In optical modulator15, the electric capacitance must be made small in order to realizehigh-speed modulation. Specifically, the electrodes are preferably madein minimum dimensions which match the diameter of incident laser light11. In this example, the upper transparent electrode and lowertransparent electrode are made to have a diameter of 30 μm, and wiringpatterns are formed in opposite directions therefrom. A lift-off methodis used for forming the electrodes. After a resist mask is formed by anoptical exposure method, an ITO film is formed by a sputtering method.

It is also necessary to reduce the electric resistance of wiresassociated with the electrodes for realizing high-speed modulation. Inthis example, a thick Cu layer having low electric resistance isdisposed on the transparent electrode wires. The thick Cu layer has athickness of 1 μm. Other than Cu, low resistance metals such as Au, Aland the like can also be used for such a thick metal layer.

In the present invention described above, the use of the aerosoldeposition method for depositing the electro-optical layer constitutesone feature of the present invention. The reason for using the aerosoldeposition method will be described below in brief.

As described in the aforementioned description of the related art andthe abstract of the invention, one object of the present invention is toprovide an ultra-high speed optical modulator which operates atfrequencies exceeding 40 GHz. For realizing a compact optical modulatorwhich maintains good modulation frequency characteristics up toultra-high frequencies, a lumped-constant load is more preferable than adistributed-constant load due to the fact that the former facilitatesimpedance matching with a driving source. The electro-optical modulatorof the present invention is a capacitive lumped-constant load. Anoptimal capacitance for matching a driving source is determined from thefrequency and the output impedance of the driving source. Thecapacitance is determined by the area of electrodes, the dielectricconstant of a medium between the electrodes (PZT that has thecomposition mentioned above has a specific dielectric constant of 100),and the thickness of the medium. As described above, the diameter of theelectrodes is determined to be 30 μm based on the diameter of the laserbeam incident on the optical modulator. Therefore, a minimum electrodearea can be calculated from the radius of the electrodes, i.e., 15 μmand the Ludolph's number. The thickness of the electro-optical layer is6 μm as mentioned above, so that the capacitance is calculated to be0.16 pF. In the optical modulator which is manufactured under theforegoing conditions, the absolute value of reactance of a load at 40GHz substantially matches the impedance of the driving source 50 Ω.

In the present state of art technologies, it is impossible to form aferroelectric transparent film having a thickness as large as 6 μm on asubstrate including dielectric substrates in arbitrary compositions,metal substrates, semiconductor substrates, resin substrates made ofplastic and polymer materials, and the like, either by a sputteringmethod or by a sol gel method, and the aerosol deposition method is theonly one which makes it possible to form such a thick ferroelectrictransparent film on the substrate. According to this deposition method,the operating frequency can be further increased because the aerosoldeposition method can deposit a ferroelectric transparent film having athickness even larger than 6 μm. It is therefore possible to create anoptical modulator which operates at frequencies exceeding 200 GHz.

FIG. 4 shows the dependence of the reflectivity on the applied voltageof optical modulator 15 in this example, where the horizontal axisrepresents the voltage, and the vertical axis represents thereflectivity. The reflectivity increases as a higher voltage is applied,and a modulation factor of approximately 10 dB can be obtained when theapplied voltage is 10 V.

[Second Example of Optical Modulator]

FIG. 5 is a cross-sectional view illustrating the structure of a secondexample of the optical modulator according to the present invention.While this optical modulator also comprises a Fabry-Perot resonator, asis the case with the optical modulator of the first example, the secondexample differs from the first example in that the upper reflective filmand electrode film are replaced with translucent metal film 55 which hasthe functions of the two films.

Lower dielectric multi-layer reflective layer 50, lower electrode layer51, lower intermediate layer 52, electro-optical layer 53, and upperintermediate layer 54 are similar in their respective compositions andforming methods to those in the first example. Translucent metal film 55is composed of two layers, a Ti layer and an Au layer. After a Ti filmof 3 nm thick is deposited, an Au film of 15 nm thick is deposited toform translucent metal film 55 in a two-layer structure composed ofTi/Au. The Ti film has the function of an adhesive layer for the Aulayer. A sputtering method is used to form their films.

FIG. 6 shows the dependence of the reflectivity on the applied currentof the optical modulator in the second example, where the horizontalaxis represents the voltage, and the vertical axis represents thereflectivity. The reflectivity increase as a higher voltage is applied,and a modulation factor of approximately 5 dB can be obtained when theapplied voltage is 10 V. While the second example exhibits a lowermodulation factor than the first example, the second example has thecharacteristic that an optical modulator can be manufacturedinexpensively because of its simple structure.

[Third Example of Optical Modulator]

FIG. 7 is a cross-sectional view illustrating the structure of a thirdexample of the optical modulator. While this optical modulator alsocomprises a Fabry-Perot resonator, as is the case with the first andsecond examples, the third example differs from the first and secondexamples in that the lower and upper reflective films and electrodefilms are replaced with translucent metal films 70, 74, respectively,each of which has the functions of the two films.

Lower intermediate layer 71, electro-optical layer 72, and upperintermediate layer 73 are similar in their respective compositions andmanufacturing methods to those of the optical modulator in the firstexample. Lower translucent metal film 70 is composed of three layers ofTi, Au, Ti, and is formed by a sputtering method. In the three layerswhich make up lower translucent metal film 70, the underlying Ti filmhas a thickness of 3 nm; the overlying Au film 6 nm; and the overlyingTi film 3 nm. Each of the Ti films has function as an adhesive layer forthe Au layer. Upper translucent metal film 74 is composed of Ti and Aulayers. After a Ti film (3 nm thick) is deposited, an Au film (12 nmthick) is deposited to form translucent metal film 55 in a two-layerstructure composed of Ti/Au. A sputtering method is used to form the Tiand Au films.

FIG. 8 shows the dependency of the reflectively on the applied voltageof the optical modulator fabricated in the third example, where thehorizontal axis represents the voltage, and the vertical axis representsthe reflectivity. The reflectivity increases as a higher voltage isapplied, and a modulation factor of approximately 4 dB can beaccomplished when the applied voltage is 10 V. While the third exampleexhibits a lower modulation factor than the first and second examples,the third example has the characteristic that an optical modulator canbe manufactured inexpensively because of its simple structure.

To clarify the effect of the lower intermediate layer, a sample of thecomponent films having different thicknesses was fabricated, and thetransmitted light spectrum was measured for the sample. The sample wascomposed of a glass substrate, a Ti film (3 nm thick), an Au film (20 nmthick), a Ti film (3 nm thick), a Ta₂O₅ film (X nm thick), and aPb(Zr0.6Ti0.4)O₃ film. Each Ti film (3 nm thick), Au film (20 nm thick),Ti film (3 nm thick), and Ta₂O₅ film (X nm thick) was deposited by asputtering method, while the Pb(Zr0.6Ti0.4)O₃ film was formed by anaerosol deposition method similar to that employed in the first example.

FIG. 9 shows the transmission spectra of the sample when the thickness Xof the Ta₂O₅ film is 10 nm and 20 nm, respectively. In transmissionspectrum 90 of the lower intermediate layer having the thickness X equalto 20 nm, vibrations due to optical interference are clearly observed.On the other hand, in transmission spectrum 91 of the lower intermediatelayer having the thickness X of 10 nm, vibrations due to opticalinterference are not clear. Consequently, the lower intermediate layerneeds a thickness X of 20 nm or more in order to form part of theresonator.

For clarifying the effect of polishing the electro-optical layer createdby the aerosol deposition method, measurements were made on changes inthe transmitted light spectrum when the electro-optical layer waspolished and when it was not polished. A sample was composed of a glasssubstrate, a Ti film (3 nm thick), an Au film (20 nm thick), a Ti film(3 nm thick), a Ta₂O₅ film (50 nm thick), and Pb(Zr0.6Ti0.4)O₃ film.Each Ti film (3 nm thick), Au film (20 nm thick), Ti film (3 nm thick),and Ta₂O₅ film (50 nm thick) was deposited by a sputtering method, whilethe Pb(Zr0.6Ti0.4)O₃ film was formed by an aerosol deposition methodsimilar to that employed in the first example.

FIG. 10 shows the transmission spectrum of the sample illustrated inFIG. 9. In transmission spectrum 100 for the polished electro-opticallayer, vibrations due to optical interference are clearly observed. Onthe other hand, in transmission spectrum 101 for the electro-opticallayer that is not polished, vibrations due to optical interference arenot clear. Consequently, the electro-optical layer created by theaerosol deposition method must be polished for forming part of aresonator.

[Fourth Example of Optical Modulator]

FIG. 11 is a cross-sectional view illustrating the structure of a fourthexample of the optical modulator according to the present invention.While this optical modulator comprises a Fabry-Perot resonator, as isthe case with the first example, the fourth example differs from thefirst example in that electro-optical layer 113 is made of a thin layerof bulk ceramics.

First, adhesive layer 110 is formed on the end face of a waveguide.Adhesive layer 110 is made of an epoxy-based resin material, therefractive index of which is the same as that of the material used toform the waveguide. Lower dielectric multi-layer film 111 and lowerelectrode layer 112 have been previously formed on electro-optical layer113 made of a thin layer of bulk ceramics. Lower dielectric multi-layerreflective layer 111 and lower electrode layer 112 are similar incomposition and manufacturing method to those in the first example. Thematerial used for electro-optical layer 113 is ceramics expressed by(Pb0.91La0.09)(Zr0.65Ti0.35)O₃, the thickness of which is reduced to 10μm by polishing.

Electro-optical layer 113, on which lower dielectric multi-layerreflective layer 111 and lower electrode layer 112 is formed, adheres toadhesive layer 110 while maintaining the parallelism. Subsequently,upper intermediate layer 114 is deposited by an ion plating method onthe surface of electro-optical layer 113 opposite to the surface whichadheres to adhesive layer 110. Upper intermediate layer 114 is formedfor purposes of adjusting the optical thickness between the reflectivelayers required to build a Fabry-Perot resonator, i.e., the sum total ofoptical thicknesses of electro-optical layer 113 and upper intermediatelayer 114. Upper intermediate layer 114 is deposited such that the sumtotal of the optical thicknesses reaches a predetermined thickness whilemeasuring the optical spectrum on a monitor during the deposition. Inthis example, upper intermediate layer 114 (58 nm thick) was laminatedon electro-optical layer 113 to form a Fabry-Perot resonator.

Finally, upper electrode layer 115 and upper dielectric multi-layerreflective layer 116 are formed on upper intermediate layer 114. Upperelectrode layer 115 and upper dielectric multi-layer reflective layer116 are similar in composition and manufacturing method to those in thefirst embodiment. However, upper electrode layer 115 is deposited at atemperature of 150° C.

When a voltage was applied to the optical modulator fabricated in thisexample, a modulation factor of 7 dB was obtained when the appliedvoltage was 5 V. In this example, while the manufacturing process iscomplicated due to the use of bulk ceramics, this example is suitablefor a device in which a high temperature process cannot be applied, andfor a composite device with an electro-optical integrated circuitbecause the manufacturing process does not include high-temperature heattreatment.

Second Embodiment

FIG. 12 illustrates the configuration of a second embodiment of aoptical modulation system according to the present invention. Laserlight 121 emitted from vertically confined surface emitting laser(VCSEL) 120 is incident on optical modulator 112. Optical modulator 122is similar in configuration and action to the first to fourth examplesof the optical modules described in the first embodiment.

Optical modulator 122 is formed on the side surface of pyramidalprotrusion 124 created on substrate 123. While substrate 123 is made ofSi, substrate 123 may be formed of quartz or the like. The shape ofpyramidal protrusion 124 can be formed by anisotropic etching of Si.Alternatively, a metal member in a pyramidal shape fabricated bycompression molding may adhere to substrate 123 to form protrusive shape124. Further alternatively, protrusive shape 124 may be formed onsubstrate 123 by direct molding of low melting point glass. VCSEL 120,optical modulator 122, and optical waveguide 125 are positioned suchthat reflected light is incident on core layer 126 of optical waveguide125 when a voltage is applied. By applying a voltage to opticalmodulator 122, the optical path can be changed to turn on and off laserlight which travels to waveguide 125.

Third Embodiment

FIG. 13 illustrates the configuration of a third embodiment of theoptical modulation system according to the present invention.

Laser light 131 emitted from vertically confined surface emitting laser(VCSEL) 130 is incident on optical modulator 132. Optical modulator 132is formed on the side surface of pyramidal protrusion 134 disposed onsubstrate 133.

The side surface of protrusion 134, on which optical modulator 132 isdisposed, is shaped into a two-dimensional concave. Optical modulator132 is in an arcuate shape, as viewed in cross-section, so that theconvex of optical modulator 132 fits the shape of the concave ofprotrusion 134. Light which has undergone optical modulation and travelstowards optical waveguide 135 is focused by the concave shape of opticalmodulator 132. The curvature of the concave shape of optical modulator132 is set so that the light image emitted by VCSEL 130 is focused onthe end surface of optical waveguide 135 and the focused size is thesame as the waveguide mode size of optical waveguide 135.

Optical modulator 132 is similar in layer structure to the first tofourth examples of the optical modulators described in the firstembodiment. A pyramidal metal member fabricated through compressionmolding can adhere to substrate 133 to form pyramidal protrusion 134.Alternatively, protrusion 134 may be formed on substrate 133 throughdirect molding of low-melting point glass.

VCSEL 130, optical modulator 132, and optical waveguide 135 arepositioned so that reflected light is incident on core layer 136 ofoptical waveguide 135 when a voltage is applied. By applying a voltageto optical modulator 132, the optical path can be changed to turn on andoff laser light which travels to waveguide 135.

Fourth Embodiment

FIG. 14 illustrates the configuration of a fourth embodiment of theoptical modulation system according to the present invention.

Laser light 141 emitted from vertically confined surface emitting laser(VCSEL) 140 is incident on optical modulator 142. Optical modulator 142is formed on the side surface of bow-shaped protrusion 144 built onsubstrate 143.

The side surface of bow-shaped protrusion 144 protrudes at an angle of45 degrees from the horizontal plane. This side surface has atwo-dimensional concave on which optical modulator 142 is formed.Optical modulator 142 is in an arcuate shape, as viewed incross-section, so that the convex of optical modulator 142 fits theconcave shape of protrusion 144. Light which has undergone opticalmodulation and travels towards optical waveguide 145 is focused by theconcave shape of optical modulator 142. The curvature of the concaveshape of optical modulator 142 is set so that a light image emitted byVCSEL 140 is focused on the end surface of optical waveguide 145 and thefocused size is the same as the mode size of optical waveguide 145.

Optical modulator 142 is similar in layer structure to the first tofourth examples of the optical modulators described in the firstembodiment. A pyramidal metal member fabricated through compressionmolding can adhere to substrate 143 to form protrusion 144 in a bowshape. Alternatively, bow-shaped protrusion 144 may be formed onsubstrate 143 through direct molding of low-melting point glass.

VCSEL 140, optical modulator 142, and optical waveguide 145 arepositioned so that reflected light is incident on core layer 146 ofoptical waveguide 145 when a voltage is applied. By applying a voltageto optical modulator 142, the optical path can be changed to turn on andoff laser light which travels to waveguide 145.

Fifth Embodiment

FIG. 15 illustrates the configuration of a fifth embodiment of theoptical modulation system according to the present invention.

Laser light 151 emitted from edge emitting laser 150 is incident onoptical modulator 152. While optical modulator 152 is similar in basicconfiguration and action to the first to fourth embodiments of theoptical modulators described in the first embodiment, optical modulator152 differs from the optical modulators in the first to fourthembodiments in that the respective layers of optical modulator 152 havedifferent thicknesses in order to build a vertically incidentFabry-Perot resonator. Optical modulator 152 is formed on the verticalend surface of optical waveguide 155, such that transmitted light fromoptical modulator 152 is incident on core layer 156 of optical waveguide155. By applying a voltage to optical modulator 152, the reflectivitycan be changed to turn on and off laser light which travels to opticalwaveguide 155.

FIG. 16 shows the dependence of the transmittance on an applied voltageof optical modulator 152 in the fifth embodiment, where the horizontalaxis represents the voltage, and the vertical axis represents thereflectivity. The transmittance increases as a higher voltage isapplied, and a modulation factor of 10 dB or more can be obtained whenthe applied voltage is 10 V.

Sixth Embodiment

FIG. 17 illustrates the configuration of an interconnection according toa sixth embodiment of the present invention. An electric signaldelivered from LSI 170 is applied to optical modulator 175. Continuouslaser light 172 emitted from vertically confined surface emitting laser(VCSEL) 171 is incident on an end region of optical waveguide 173.Optical waveguide 173 is formed on substrate 174. The end region ofoptical waveguide 173 is processed to taper at an angle of 45 degrees,and optical modulator 175 is formed on the tapered surface.

By modulating the voltage of the output signal supplied from LSI 170 tooptical modulator 175, the optical path can be changed to turn on andoff the laser light applied to optical waveguide 173. Optical modulator175 is configured to reflect laser light from optical waveguide 173 whena voltage is applied, and the position of optical modulator 175 is setso that the reflected light is incident on the core layer of opticalwaveguide 173. Optical modulator 175 is similar in configuration andaction to the first to fourth examples of the optical modulatorsdescribed in the first embodiment.

The other end region of optical waveguide 173 opposite to the end regionon which optical waveguide 173 is formed is also processed to taper atan angle of 45 degrees. This tapered end surface induces a 90 degreechange in the optical path of laser light on which a signal ismultiplexed and which travels through optical waveguide 173. Laser lightemitted from the other end of optical waveguide 173 is incident onphoto-detector 176, and is converted to an electric signal byphoto-detector 176. The electric signal opto-electrically converted byphoto-detector 176 is applied to LSI 177.

According to the interconnection configured as described above, anelectric signal (information) can be converted to an optical signal andcan be transmitted between LSI 170 and LSI 177. The transmission ofinformation through an optical signal enables high-speed communicationsusing optical pass which are immune to disturbances caused byelectromagnetic noise.

When the optical modulator of the present invention described above, anda optical modulation system incorporating the optical module is used inan optical communication device, particularly, an optical transmitter inan optical fiber communication system and a spatial propagation opticalcommunication system, it is possible to provide an optical communicationsystem at high speeds in a range of 40 to 200 Gbps.

While preferred embodiments of the present invention have been describedusing specific terms, such description is for illustrative purposesonly, and it is to be understood that changes and variations may be madewithout departing from the spirit or scope of the following claims.

1. A optical modulation system comprising: a light source; an opticalmodulator driven by an external driving source for modulating lightemitted from said light source; and an optical waveguide for guidingmodulated light modulated by said optical modulator, wherein saidoptical modulator is a Fabry-Perot resonator type optical modulatorformed on a substrate, said optical modulator includes a laminatestructure comprised of an electro-optical layer, an upper electrodelayer overlying the electro-optical layer and a lower electrode layerunderlying the electro-optical layer to sandwich the electro-opticallayer therebetween, and an upper reflective layer overlying the upperelectrode layer and a lower reflective layer underlying the lowerelectrode layer to sandwich the upper electrode, electro-optical layer,and lower electrode layer therebetween, and said optical modulator has areactance, an absolute value of which is higher than an impedance ofsaid external driving source in a frequency range equal to or lower than200 GHz.
 2. The optical modulation system according to claim 1, wherein:said optical waveguide has an end surface inclined with respect to anoptical axis of said optical waveguide, said optical modulator is formedon an inclined surface, and said light source is disposed such that thelight emitted therefrom is transmitted through a side surface of saidoptical waveguide and is incident on said optical modulator.
 3. Theoptical modulation system according to claim 1, wherein: said opticalwaveguide has an end surface vertical to an optical axis of said opticalwaveguide, said optical modulator is formed on a vertical surface, andsaid light source is disposed such that the light emitted therefrom isincident on said optical modulator at right angles.
 4. The opticalmodulation system according to claim 1, further comprising a lens forcoupling the light emitted from said light source to waveguided light ofsaid optical waveguide.
 5. The optical modulation system according toclaim 1, further comprising an optical modulator support having areflective surface for converting the emission optical axis of saidlight source to an optical axis of said optical waveguide, wherein saidoptical modulator is formed on the reflective surface of said opticalmodulator support.
 6. The optical modulation system according to claim5, wherein said reflective surface is concave shaped.
 7. The opticalmodulation system according to claim 6, wherein said concave has acurvature for focusing light emitted from said light source on saidoptical waveguide.
 8. A optical modulation system comprising: a lightsource; an optical modulator for modulating light emitted from saidlight source; and an optical waveguide for guiding modulated lightmodulated by said optical modulator, wherein said optical modulator is aFabry-Perot resonator type optical modulator formed on a substrate, saidoptical modulator includes a laminate structure comprised of anelectro-optical layer, an upper electrode layer overlying theelectro-optical layer and a lower electrode layer underlying theelectro-optical layer to sandwich the electro-optical layertherebetween, and an upper reflective layer overlying the upperelectrode layer and a lower reflective layer underlying the lowerelectrode layer to sandwich the upper electrode, electro-optical layer,and lower electrode layer therebetween, and said electro-optical layerhas a linear electro-optical effect.
 9. The optical modulation systemaccording to claim 8, wherein: said optical waveguide has an end surfaceinclined with respect to the optical axis of said optical waveguide,said optical modulator is formed on the inclined surface, and said lightsource is disposed such that the light emitted therefrom is transmittedthrough a side surface of said optical waveguide and is incident on saidoptical modulator.
 10. The optical modulation system according to claim8, wherein: said optical waveguide has an end surface vertical to anoptical axis of said optical waveguide, said optical modulator is formedon a vertical surface, and said light source is disposed such that thelight emitted therefrom is incident on said optical modulator at rightangles.
 11. The optical modulation system according to claim 8, furthercomprising a lens for coupling the light emitted from said light sourceto waveguided light of said optical waveguide.
 12. The opticalmodulation system according to claim 8, further comprising an opticalmodulator support having a reflective surface for converting theemission optical axis of said light source to the optical axis of saidoptical waveguide, wherein said optical modulator is formed on thereflective surface of said optical modulator support.
 13. The opticalmodulation system according to claim 12, wherein said reflective surfaceis concave shaped.
 14. The optical modulation system according to claim13, wherein said concave has a curvature for focusing light emitted fromsaid light source on said optical waveguide.
 15. An optical interconnectdevice having an optical modulation system, said optical modulationsystem comprising: a substrate; a light source having an emissionoptical axis vertical to the surface of said substrate; an opticalwaveguide having an optical axis parallel with the surface of saidsubstrate; and an optical modulator driven by an external driving sourcefor modulating light emitted from said light source, wherein saidoptical modulator is a Fabry-Perot resonator type optical modulatorformed on a substrate, said optical modulator includes a laminatestructure comprised of an electro-optical layer, an upper electrodelayer overlying the electro-optical layer and a lower electrode layerunderlying the electro-optical layer to sandwich the electro-opticallayer therebetween, and an upper reflective layer overlying the upperelectrode layer and a lower reflective layer underlying the lowerelectrode layer to sandwich the upper electrode, electro-optical layer,and lower electrode layer therebetween, and said optical modulator has areactance, the absolute value of which is higher than the impedance ofsaid external driving source in the frequency range equal to or lowerthan 200 GHz, said optical waveguide has an end surface inclined withrespect to the optical axis of said optical waveguide, and said opticalmodulator is formed on the inclined surface, and said light source isdisposed such that the light emitted therefrom is transmitted through aside surface of said optical waveguide and is incident on said opticalmodulator.
 16. An optical interconnect device having a opticalmodulation system, said optical modulation system comprising: asubstrate; a light source having an emission optical axis vertical tothe surface of said substrate; an optical waveguide having an opticalaxis parallel with the surface of said substrate; and an opticalmodulator for modulating light emitted from said light source, whereinsaid optical modulator is a Fabry-Perot resonator type optical modulatorformed on a substrate, said optical modulator includes a laminatestructure comprised of an electro-optical layer, an upper electrodelayer overlying the electro-optical layer and a lower electrode layerunderlying the electro-optical layer to sandwich the electro-opticallayer therebetween, and an upper reflective layer overlying the upperelectrode layer and a lower reflective layer underlying the lowerelectrode layer to sandwich the upper electrode, electro-optical layer,and lower electrode layer therebetween, and said electro-optical layerhas a linear electro-optical effect, said optical waveguide has an endsurface inclined with respect to the optical axis of said opticalwaveguide, and said optical modulator is formed on the inclined surface,and said light source is disposed such that the light emitted therefromis transmitted through a side surface of said optical waveguide and isincident on said optical modulator.
 17. An optical interconnect devicehaving a optical modulation system, said optical modulation systemcomprising: a substrate; a light source having an emission optical axisvertical to the surface of said substrate; an optical waveguide havingan optical axis parallel with the surface of said substrate; an opticalmodulator support having a reflective surface for converting theemission optical axis of said light source to the optical axis of saidoptical waveguide; and an optical modulator driven by an externaldriving source and formed on the reflective surface of said opticalmodulator support for modulating light emitted from said light source,wherein said optical modulator is a Fabry-Perot resonator type opticalmodulator formed on a substrate, said optical modulator includes alaminate structure comprised of an electro-optical layer, an upperelectrode layer overlying the electro-optical layer and a lowerelectrode layer underlying the electro-optical layer to sandwich theelectro-optical layer therebetween, and an upper reflective layeroverlying the upper electrode layer and a lower reflective layerunderlying the lower electrode layer to sandwich the upper electrode,electro-optical layer, and lower electrode layer therebetween, and saidoptical modulator has a reactance, the absolute value of which is higherthan the impedance of said external driving source in the frequencyrange equal to or lower than 200 GHz.
 18. An optical interconnect devicehaving a optical modulation system, said optical modulation systemcomprising: a substrate; a light source having an emission optical axisvertical to the surface of said substrate; an optical waveguide havingan optical axis parallel with the surface of said substrate; an opticalmodulator support having a reflective surface for converting theemission optical axis of said light source to the optical axis of saidoptical waveguide; and an optical modulator formed on the reflectivesurface of said optical modulator support for modulating light emittedfrom said light source, wherein said optical modulator is a Fabry-Perotresonator type optical modulator formed on a substrate, said opticalmodulator includes a laminate structure comprised of an electro-opticallayer, an upper electrode layer overlying the electro-optical layer anda lower electrode layer underlying the electro-optical layer to sandwichthe electro-optical layer therebetween, and an upper reflective layeroverlying the upper electrode layer and a lower reflective layerunderlying the lower electrode layer to sandwich the upper electrode,electro-optical layer, and lower electrode layer therebetween, and saidelectro-optical layer has a linear electro-optical effect.
 19. Anoptical communication device having a optical modulation system, saidoptical modulation system comprising: a substrate; a light source havingan emission optical axis vertical to the surface of said substrate; anoptical waveguide having an optical axis parallel with the surface ofsaid substrate; and an optical modulator driven by an external drivingsource for modulating light emitted from said light source, wherein saidoptical modulator is a Fabry-Perot resonator type optical modulatorformed on a substrate, said optical modulator includes a laminatestructure comprised of an electro-optical layer, an upper electrodelayer overlying the electro-optical layer and a lower electrode layerunderlying the electro-optical layer to sandwich the electro-opticallayer therebetween, and an upper reflective layer overlying the upperelectrode layer and a lower reflective layer underlying the lowerelectrode layer to sandwich the upper electrode, electro-optical layer,and lower electrode layer therebetween, and said optical modulator has areactance, the absolute value of which is higher than the impedance ofsaid external driving source in the frequency range equal to or lowerthan 200 GHz, said optical waveguide has an end surface inclined withrespect to the optical axis of said optical waveguide, and said opticalmodulator is formed on the inclined surface, and said light source isdisposed such that the light emitted therefrom is transmitted through aside surface of said optical waveguide and is incident on said opticalmodulator.
 20. An optical communication device having a opticalmodulation system, said optical modulation system comprising: asubstrate; a light source having an emission optical axis vertical tothe surface of said substrate; an optical waveguide having an opticalaxis parallel with the surface of said substrate; and an opticalmodulator for modulating light emitted from said light source, whereinsaid optical modulator is a Fabry-Perot resonator type optical modulatorformed on a substrate, said optical modulator includes a laminatestructure comprised of an electro-optical layer, an upper electrodelayer overlying the electro-optical layer and a lower electrode layerunderlying the electro-optical layer to sandwich the electro-opticallayer therebetween, and an upper reflective layer overlying the upperelectrode layer and a lower reflective layer underlying the lowerelectrode layer to sandwich the upper electrode, electro-optical layer,and lower electrode layer therebetween, and said electro-optical layerhas a linear electro-optical effect, said optical waveguide has an endsurface inclined with respect to the optical axis of said opticalwaveguide, and said optical modulator is formed on the inclined surface,and said light source is disposed such that the light emitted therefromis transmitted through a side surface of said optical waveguide and isincident on said optical modulator.
 21. An optical communication devicehaving a optical modulation system, said optical modulation systemcomprising: a substrate; a light source having an emission optical axisvertical to the surface of said substrate; an optical waveguide havingan optical axis parallel with the surface of said substrate; an opticalmodulator support having a reflective surface for converting theemission optical axis of said light source to the optical axis of saidoptical waveguide; and an optical modulator driven by an externaldriving source for modulating light emitted from said light source,wherein said optical modulator is a Fabry-Perot resonator type opticalmodulator formed on a substrate, said optical modulator includes alaminate structure comprised of an electro-optical layer, an upperelectrode layer overlying the electro-optical layer and a lowerelectrode layer underlying the electro-optical layer to sandwich theelectro-optical layer therebetween, and an upper reflective layeroverlying the upper electrode layer and a lower reflective layerunderlying the lower electrode layer to sandwich the upper electrode,electro-optical layer, and lower electrode layer therebetween, and saidoptical modulator has a reactance, the absolute value of which is higherthan the impedance of said external driving source in the frequencyrange equal to or lower than 200 GHz.
 22. An optical communicationdevice having a optical modulation system, said optical modulationsystem comprising: a substrate; a light source having an emissionoptical axis vertical to the surface of said substrate; an opticalwaveguide having an optical axis parallel with the surface of saidsubstrate; an optical modulator support having a reflective surface forconverting the emission optical axis of said light source to the opticalaxis of said optical waveguide; and an optical modulator formed on thereflective surface of said optical modulator support for modulatinglight emitted from said light source, wherein said optical modulator isa Fabry-Perot resonator type optical modulator formed on a substrate,said optical modulator includes a laminate structure comprised of anelectro-optical layer, an upper electrode layer overlying theelectro-optical layer and a lower electrode layer underlying theelectro-optical layer to sandwich the electro-optical layertherebetween, and an upper reflective layer overlying the upperelectrode layer and a lower reflective layer underlying the lowerelectrode layer to sandwich the upper electrode, electro-optical layer,and lower electrode layer therebetween, and said electro-optical layerhas a linear electro-optical effect.